1. Field of the Invention
The present invention relates to a tubular radiation absorbing device for solar heating applications, especially for a parabolic collector in a solar heat collecting apparatus, which comprises a central tube, a glass tubular jacket surrounding the central tube so as to form a ring-shaped space between the tubular jacket and the central tube, at least one hydrogen window and an expansion compensating device connecting the central tube and the glass tubular jacket, so that they can move relative to each other in a longitudinal direction.
2. Description of the Related Art
Tubular radiation absorbing devices or absorber pipes are used in parabolic trough collectors to utilize solar radiation. The solar radiation is concentrated by a tracking mirror on a tubular radiation absorbing device and converted into heat. The heat is conducted away by a heat-carrying medium passing through the tubular radiation absorbing device and is used directly as process heat or converted into electrical energy.
A tubular radiation absorbing device with a central tube and a glass tubular jacket surrounding the central tube is known from DE 102 31 467 B4. A glass-metal transitional element is arranged on the free end of the glass tubular jacket. The central tube and the glass-metal transitional element are connected with each other by means of at least one expansion compensating device, so that they are movable relative to each other in the longitudinal direction. The expansion compensating device is arranged at least partially in an annular space between the central tube and the glass-metal transitional element. The expansion compensating device has two functions, namely, compensation for length changes due to thermal expansion of the central metal pipe and the glass tubular jacket and, at the same time, protection of the glass-metal transitional element from radiation reflected from the central metal pipe, so that no overheating can occur in this region and the vacuum-tight seal between the central tube and the glass tubular jacket is not compromised.
Because the expansion compensating device is not arranged in an axial direction next to or following the glass-metal transitional element, but under the glass-metal transitional element (i.e. next to or beside it in the radial direction), the structure is considerably shortened and at the same time reduces the shaded or blocked area of the tubular radiation absorbing device, which increases the performance of the tubular radiation absorbing device. A compact structure comprising the expansion compensating device and the glass-metal transitional element is thus formed, which guarantees a vacuum-tight seal in a simple way and makes unnecessary additional structural components, such as an interior diaphragm. For example, the expansion compensating device comprises a folding bellows.
The tubular radiation absorbing device requires a working temperature of between 300° C. and 400° C. for solar power generation. Thermo-oil flows through the tubular radiation absorbing device.
Free hydrogen, which dissolves in the thermo-oil, is generated by aging of the thermo-oil. This hydrogen reaches the evacuated ring-shaped space between the central tube and the glass tubular jacket by permeation through the central tube. The permeation rate increases with increasing operating temperature, whereby the pressure in the ring-shaped space also increases. This pressure increase leads to increased heat losses and a reduction of the efficiency of the tubular radiation absorbing device.
Suitable steps or measures are required to maintain the vacuum in the ring-shaped space. One such measure comprises elimination of the hydrogen in the ring-shaped space by combining it with. a suitable material. A getter is used for this purpose.
A getter arrangement is described in WO 2004/063640 A1, in which a getter bar is arranged in the ring-shaped space between the central tube and the tubular jacket. This arrangement has the disadvantage that the bar is in a region, which is exposed to direct radiation. Especially the getter bar could be heated by radiation coming from the mirror that misses the central tube or only glances off it and is reflected to a large extent. Since the getter bar is almost completely thermally isolated from the central tube and the tubular jacket in a vacuum, the temperature of the bar and thus the getter strongly fluctuates depending on the amount of incident radiation. Since the getter material with a given loading degree or content has a temperature-dependent equilibrium pressure (equilibrium between gas desorption and adsorption), temperature fluctuations of the getter lead to undesirable pressure fluctuations. Generally the characteristic parameters for getter materials are the adsorption rate and the equilibrium pressure. Both parameters increase generally with increasing temperature. A further disadvantage is that cylindrical press blanks are used, which decompose into a powdery state at higher hydrogen content. The powder distributes itself in the evacuated intervening space during use of the bar and that leads to impairment of the radiation transmission through the tubular jacket.
Another measure comprises removal of the hydrogen by conducting into the outside atmosphere. For example, EP 0 286 281 A1 describes insertion of a membrane of a material that has a high porosity for hydrogen and a low permeation rate for other gases between the vacuum and the outer atmosphere. The hydrogen can escape through this membrane into the outer atmosphere, without gases from the atmosphere entering into the vacuum. For this purpose the glass tubular jacket is provided with a tubular window, which is closed by means of a hydrogen-permeable palladium or palladium alloy window. A palladium tubule can be used for this window, which extends into the ring-shaped space. Alternatively the folding bellows, which is used as the expansion compensating device, can be coated with palladium.
A hydrogen window of this sort is generally exposed directly to incident solar radiation. Local heating of the glass tubular jacket, which can lead to breakage of glass, occurs because of strong heating by the solar radiation. It is known from experience with solar power plants in California that a failure rate of about 5% occurs because of this problem. Furthermore a hydrogen window that is arranged on the exterior of the tubular radiation absorbing device is exposed to exterior environmental influences like rain or dirt. Corrosion occurs because of exposure to rain water, which results in the destruction of the entire hydrogen window.
Also if the tube operator thus fails to uncover the hydrogen window, it can no longer perform its intended function.